THE SENSE REACTIONS OF FLIES. 149 



Covered foods offered to house-flies (unconfined). 



In the experiments testing covered and uncovered foods a series of preliminary 

 experiments with free Musca was made to find out the effect of covering food in 

 various ways ; the results obtained were further tested by several series of 

 experiments, in which different foods were covered in the same way. 



In all the experiments the behaviour of the flies was noted, and the numbers 

 which came to feed were counted from time to time. 



In the preliminary experiments sight tests were made (a) by using covers of 

 transparent materials (glass, talc, mica, celluloid, and gelatine) in order to test the 

 reactions of the flies to varying degrees of opacity, through which it was presumed 

 the smell could not penetrate (none could be detected by me) ; and (b) by using 

 covers of muslin, through which the smell could pass. 



The following results were obtained. In the controls, with uncovered baits, very 

 many flies came to feed. 



(a) When the baits were covered by glass plates, a few flies settled upon them as 

 a rule, and sometimes appeared to be trying to get at the bait, especially when it 

 was touching the glass. They never came to these covered baits, however, in the 

 same proportion as to the uncovered ones. They also appeared able to distinguish 

 between dishes containing baits and empty dishes, when both were covered by glass ; 

 for when empty dishes were exposed, the flies did not, as a rule, come to them at 

 all. Very similar results were obtained when the baits were inserted inside blown 

 glass bulbs and placed among confined flies, and also when they were covered by 

 talc, mica, gelatine and celluloid. 



(b) When the baits were covered with muslin, more flies settled upon these covers 

 than on the glass, and the flies generally appeared to be trying to get at the food 

 inside. They seemed able to distinguish between attractive and repellent foods 

 so covered ; for in the latter case, those circling round the baits never settled. 



Free blow-flies also were seen to behave in a similar way. They were strongly 

 attracted to decaying meat covered by muslin, coming to it from a considerable 

 distance. 



In the subsequent series, various foods and other substances, covered in the same 

 way, were exposed at the same time, to see whether the flies could distinguish 

 between the different contents of the dishes. When the baits were covered by glass, 

 celluloid, or gelatine, the flies settled upon all, although larger numbers came to the 

 covers of the dishes containing attractive foods. 



When muslin covers were used, the flies appeared to be able to distinguish more 

 easily between the contents of the dishes, since many more flies settled upon the 

 covers of those containing attractive foods, although a few settled for a short time 

 upon all, except those containing very repellent substances, e.g., oils, etc., to which 

 none came. 



Confined house-flies with painted eyes. 



In these experiments the eyes of the flies were coated over so as to prevent their 

 seeing what substances were fed to them and thus test the intensity of their sense 

 of smell. 



